Fingerprint sensing devices have been widely used for security systems and fingerprint authentication. A typical fingerprint sensing device includes some form of a sensor for generating a representation of a fingerprint of a finger that is placed over or in direct contact with the sensor. In general, fingerprint detection methods may be capacitive, thermal, optical and ultrasonic; wherein the sensor captures the fingerprint image when a finger is either statically placed on the surface of the sensor or slid over the sensor surface.
More recently, electronic devices such as computers, and portable electronic devices like cell phones, have incorporated a fingerprint detector for individual authentication for security and data protection. For such devices, a main challenge is the size of the sensor, since it is more desirable to have a sensor with a small footprint. In addition, product reliability and system robustness are also key requirements.
In some conventional embodiments, capacitive based fingerprint sensing devices have been widely implemented. A capacitive based fingerprint sensing device may include a two dimensional electrode array which creates a capacitance between each electrode on the substrate and a grounded finger that is placed on the surface of the array. A two dimensional capacitance map of the pattern of ridges of the finger representing the fingerprint image is produced. However, a disadvantage of such approach is that the die size of the capacitive based sensor must be large enough to provide sufficient contact area for the finger to touch; therefore it is not an ideal candidate for use on a small sized handheld device which provides only a limited allowable foot print. Another drawback of such a capacitive based sensor is the issue of electrostatic discharge or ESD. The electrostatic discharge from the human body through the finger can damage the electrode. Additionally, it can be a challenge in acquiring a good quality image when the dryness or moisture level of the finger surface varies; the degree of moisture on the finger surface can cause the fingerprint recognition system to output erroneous results.
In another conventional embodiment, a fingerprint sensing device may consist of a linear array capacitive image sensor, a capacitive based rate sensor, and a sensor circuit for processing a fingerprint image when a finger is placed in direct contact with the capacitive sensor. The rate sensor is incorporated to sense the speed of the finger and provides rate drive signals to the sensor circuit and further combines image signals and rate signals to generate a fingerprint image. Since such system is based on the capacitive method, most of the above mentioned limitations which are associated with capacitive type sensor devices are unavoidable.
Another conventional optical fingerprint sensing device is known to utilize light reflected from a surface of a finger placed on an image sensing module to obtain finger print images. This image sensing device detects the movements of the finger, captures multiple fingerprint images, and subsequently processes these images to generate a complete finger print image. Accordingly, such an approach employs complicated image processing methods and requires high processing power in order to obtain an image quality needed to generate a useful fingerprint image for enabling fingerprint authentication. Alternatively, other conventional optical type fingerprint sensing devices may include an extra finger movement detector; for example, an optical image sensing module with a movement detecting function such as a roller has also been introduced. However, due to the large size of such module, complexity in its structure and lack of product robustness, such method is not suitable for assembly on a small size portable electronics device.